Ischaemia

• Ischaemia is the failure or reduction of arterial blood supply to an organ or tissue.

Causes

• Arteriospasm
  • Arterial trauma
  • Ergotism
  • Other drug effects
• Compression of arteries/ arterioles
  • Ligatures
  • Tourniquets
• Thromboembolism
  • Vascular impaction by embolic fragments

Sequelae

• Several factors determine the extent and consequences of arterial obstruction.
  1. Degree of occlusion
     • May be partial or complete.
  2. Rapidity of occlusion
     • May be sudden or gradual.
  3. Presence and extent of a collateral circulation, such as
     • Anastomoses
     • Whether end areterioles are functional or actual.
     • Efficient alternative arterial supplies.
  4. Vulnerability of tissues/ cells to ischemia.
     • For example, their oxygen dependency and metabolic rate.
       • E.g. neurones and cardiac myocytes are much more susceptible than fibrous tissue.

• There are two possible scenarios in ischeamia, each with their own sequelae.
  1. Complete, sudden failure of blood supply
     • Unless there is a good alternative collateral supply, this results in necrosis of tissue.
       • Collateral supplies may be inadequate to compensate if the blockage is severe.
     • This focal type of lesion is known as infarction.
  2. Incomplete, gradual failure of blood supply
     • Results in “accommodation”.
       • Manifests as atrophy, sometimes with fibrosis, following cellular hypoxia.
         • Minimal fibrosis occurs if collateral/ alternative supplies are good.